MicroRNAs are small regulatory RNAs present in all animal cells. Many microRNAs have been shown to be involved in human disease. Information about the disease status of a patient can be obtained by quantitative analysis of the levels of specific microRNAs in normal and diseased tissue, or in body fluids such as blood serum/plasma, cerebrospinal fluid, or urine. Detection of circulating miRNAs in blood plasma or other biological fluids promises to provide a convenient, inexpensive, and noninvasive way to diagnose and/or track the progress of treatment of diseases that are currently difficult to diagnose or successfully treat.